Using secondary lymphocyte proliferative responses and secondary cell-mediated cytotoxicity we have continued to probe the complexities of the alloantigens encoded in the HLA region. The discovery of a new gene "SB" in our laboratory has fostered a large number of collaborations and rapid progress has been made in defining the genetics, structure and function of the SB antigens. A monoclonal antibody and alloantisera have been identified which react with the gene product. Initial structural studies indicate that the SB molecule has the structural characteristics typical of Ia-like antigens, and has amino acid sequence homology to HLA-DR (and H-2E). The genetics of this locus has been explored in a number of populations in USA, Europe and the Orient. Furthermore, T cell cloning has facilitated definition of these antigens. Analysis of other complexities of HLA markers has allowed the definition of two antigens intimately associated with Bw44. These antigens are of particular interest since they shed new light on linkage disequilibrium of the Bw44-related antigens, and they may have important biological relevance since they appear to regulate the level of expression of the Bw44 antigen on platelets.